Letter
Emergence of Vancomycin-Intermediate
Staphylococcus aureus and S. sciuri, Greece
To the Editor: Staphylococcal isolates with reduced susceptibility
to glycopeptides, such as vancomycin and teicoplanin, are a serious
public health problem because staphylococci frequently show multidrug
resistance, and glycopeptides are the only remaining effective drugs.
Since the early reports of glycopeptide-resistant staphylococci,
teicoplanin resistance has become more common than vancomycin resistance,
particularly among coagulase-negative staphylococcal species (1–3).
In cases of staphylococci with reduced susceptibility to vancomycin
(vancomycin-intermediate staphylococci), an increasing number of
strains showing heteroresistance are reported (strains that contain
subpopulations of cells at frequencies >10-6
for which the vancomycin MICs are 8 µg/mL to 16 µg/mL); homogeneous
resistance still appears to be rare (2,4–7). In
northern Greece, resistance to teicoplanin has recently been documented
in S. haemolyticus strains isolated from clinical infections
(8). We report the first bloodstream infections
in Greece associated with S. aureus and S. sciuri strains
that have homogeneous intermediate-resistance to vancomycin (MIC
= 8 µg/mL).
In our department, all clinically significant staphylococcal isolates
are screened for reduced susceptibility to vancomycin and teicoplanin
by an agar incorporation method (9), which has
been routinely performed since January 1999. An inoculum of 104
CFU/spot from a log-phase broth culture was spread on Mueller-Hinton
agar plates containing appropriate antibiotic concentrations. The
strains were incubated for a full 24 hours before the MICs were
read. When a reduced susceptibility to vancomycin was observed (MIC
8 to 16 µg/mL), the test was repeated for confirmation of the result
and the strains were also tested by National Committee for Clinical
Laboratory Standards (NCCLS) broth microdilution (9)
and E-test (AB Biodisk, Solna, Sweden) with BHI agar (Oxoid, Ltd.,
Basingstoke, Hampshire, UK) and an inoculum density adjusted to
0.5 McFarland value. S. aureus ATCC 29213, which had MICs
for vancomycin of 1 µg/mL and for teicoplanin of 0.5 µg/mL, was
used as a control for the estimation of the MICs. Two vancomycin-intermediate
staphylococcal isolates (one S. aureus and one S. sciuri)
were recovered in our hospital during December 2000 and April 2001,
respectively. The organisms were identified with the Vitek system
(bioMerieux Vitek, La Balme les Grottes, France). Slide-coagulase
test and Staph ID 32 API system (API system, bioMerieux) confirmed
identification. Susceptibility to 18 antimicrobial agents was evaluated
with the Vitek system according to the recommendations of the manufacturer,
and carriage of the mecA gene was confirmed with a polymerase
chain reaction (PCR) that amplifies a 449-bp product.
The first strain (S. aureus) was recovered from a 52-year-old
man who was hospitalized after a severe traffic accident. The patient
had multiple injuries, including an external laryngeal trauma, pelvic
ring disruption, and various fractures of the extremities. He underwent
immediate tracheotomy, and a neurosurgical operation was performed
to evacuate an extracerebral hematoma. Ceftazidime, clindamycin,
ciprofloxacin, metronidazole, teicoplanin, and vancomycin were periodically
administered as prophylaxis. An oxacillin-resistant S. aureus
isolate was recovered from two blood cultures 4 weeks after
the patient’s admission. The strain was also resistant to tobramycin,
macrolides, tetracyclines, rifampicin, and fusidic acid, and had
intermediate resistance to vancomycin (MIC 8 µg/mL) and teicoplanin
(MIC 16 µg/mL) by all tested methods (agar dilution, broth microdilution,
and E-test). The strain was susceptible to chloramphenicol, cotrimoxazole,
fosfomycin, gentamicin, kanamycin, nitrofurantoin, and ofloxacin.
The removal of an intravenous catheter and treatment with gentamicin
and vancomycin eradicated the infection.
The second strain (S. sciuri) was recovered from a 35-year-old
man who was an intravenous drug user. He was admitted with renal
failure, electrolyte disturbances, and acute respiratory distress,
which necessitated intubation and mechanical ventilation. The patient
became febrile, and multiple courses of antibiotics (amikacin, cefepime,
ciprofloxacin, metronidazole, and vancomycin, alone or in combinations)
were administered before the S. sciuri strain was isolated.
Seven weeks after his admission, an oxacillin-resistant S. scuiri
strain that had cross-resistance to aminoglycosides, macrolides,
quinolones, rifampicin, and tetracycline was found in subsequent
blood cultures. The MIC of the strain for vancomycin was 8 µg/mL
and for teicoplanin 16 µg/mL by the agar dilution method, and the
result was confirmed by the E-test and the broth microdilution method.
The strain was susceptible only to cotrimoxazole, fosfomycin, and
nitrofurantoin. The patient improved clinically and was subsequently
discharged on cotrimoxazole and vancomycin therapy.
In both cases, the MICs of vancomycin remained stable after repeated
subcultures in a drug-free medium. PCR amplification showed that
both staphylococcal strains carried the mecA gene. However,
the vanA, vanB, and vanC genes were not amplified
in any strain.
Vancomycin-intermediate staphylococci have been sporadically reported
from clinical infections after prolonged exposure to vancomycin
or preexisting infection with methicillin-resistant staphylococci
(5,7). In our hospital, high rates of methicillin-resistant
staphylococci are detected, and vancomycin has been the only treatment
uniformly effective against staphylococcal infections. However,
this is the first report of infection caused by vancomycin-intermediate
S. aureus in Greece. In addition, S. sciuri, a species
considered taxonomically the most primitive among staphylococci
and found primarily in rodents and primitive mammals, has not been
implicated previously in human infections caused by vancomycin-intermediate
strains in our region or elsewhere.
Although various studies have described staphylococci with reduced
susceptibility to vancomycin, the existence of isolates that have
the homogeneous vancomycin-intermediate phenotype is rather limited
(4–7). In this report, the vancomycin MIC for both
staphylococcal isolates was repeatedly 8 µg/mL, and a confluent
growth was observed after 24 hours on Mueller Hinton agar containing
vancomycin at a concentration of 4 µg/mL. Discrete colonies were
detected only in plates containing 6 µg/mL of vancomycin but
not in plates containing 8 µg/mL of the drug even when a prolonged
incubation of 48 hours and an inoculum of 106 CFU/spot
were used. The Vitek system recorded correctly both isolates as
having intermediate level of resistance to glycopeptides; this result
was particularly important given the wide use of this commercial
system in many hospital laboratories. However, as was reported for
previous glycopeptide-intermediate staphylococci (2,5),
both isolates appeared to be susceptible to vancomycin when tested
by the disk diffusion method, with zones of 15 mm and 16 mm for
the S. aureus and the S. sciuri isolates, respectively.
Vancomycin-resistant staphylococci had not been detected in our
hospital until December 2000. Therefore, the emergence of vancomycin-resistant
staphylococci is a recent development, suggesting a potential for
wider dissemination. Since the NCCLS agar dilution method we used
is not sensitive for the detection of heterogeneous resistance phenotypes
(7), screening for vancomycin-resistant subpopulations
in the vancomycin-susceptible isolates (mainly those with MIC for
vancomycin of 4 µg/mL) is important. Whether the mechanisms responsible
for homogeneous intermediate resistance to vancomycin in our staphylococci
are similar to those described in isolates from Japan and elsewhere
still remains to be answered.
Athanassios Tsakris,*† Ekaterini Papadimitriou,‡ John Douboyas,‡
Fotini Stylianopoulou,† and Evangelos Manolis†
*University of Thessaloniki, Thessaloniki, Greece; †University
of Athens, Athens, Greece; and ‡AHEPA University Hospital, Thessaloniki,
Greece
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